In gas turbines a burner is typically provided with a premixing chamber, in which a fuel, in particular in gaseous form, is mixed with air, in order then to combust the resultant mixture. The efficiency of the gas turbine and also the formation of undesired emission products, in particular nitrogen oxides, are in this case essentially dependent on proper mixing of the fuel with the air.
In particular in a gas turbine operated with natural gas, the natural gas is frequently injected radially, i.e. perpendicular to the direction of air flow (the “jet in crossflow” method). In this way, appropriate mixing of natural gas and air can be achieved.
“Vortex generators” are additionally known for mixing, for example from DE 44 26 351 A1, in which a combustion chamber is disclosed which consists substantially of a first stage and a second stage arranged downstream in the direction of flow. In this case, the first stage comprises a mixer on the head side for forming a fuel/air mixture and vortex generators are present on the outflow side of the mixer. These serve in particular to swirl hot air, which is then guided into a premixing zone for mixing with fuel and then into a combustion zone of the second stage.
It is additionally known to operate gas turbines with a plurality of fuels, for example with natural gas and hydrogen. Operation may proceed either simultaneously with a plurality of fuels or with just one of the fuels. This in particular increases the flexibility of the gas turbine, since fuels may be used according to availability. However, the different operating modes also result in additional requirements for the gas turbine and the burners thereof.
When using or admixing hydrogen, however, there is an increased risk, over the use of just natural gas, of flareback and self-ignition. To reduce this risk, it is possible to add a less reactive gas to the hydrogen to form a hydrogen-containing fuel gas, which frequently however disadvantageously has a lower energy density than natural gas. It is thus in particular necessary to design injection so as to be suitable to accommodate different volumetric flow rates for the different fuels.
To reduce further the risk of flareback and self-ignition, it is known to inject the hydrogen-containing fuel gas in a coaxial direction, i.e. in the direction of air flow. In this way, an air-side pressure drop is in particular also reduced, this being greater when hydrogen-containing fuel gas is used, due to the greater volumetric flow rate, than when natural gas is used.
EP 2 604 919 A1 for example discloses a fuel nozzle for two fuels, with an inner pipe with radially oriented outlet orifices for a first fuel and with an outer pipe surrounding the inner pipe and having axially oriented outlet orifices for a second fuel. Using such a nozzle it is possible, for example, to inject a hydrogen-containing fuel gas in the direction of air flow via the axial outlet orifices. To improve mixing of the fuel gas with the air, a “lobe mixer” is additionally provided. The second fuel, for example natural gas, is then injected via the radial outlet orifices. A disadvantage here is that there are no further options for optimizing mixing in particular of the natural gas and the air.